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A federally funded 10-year study has found that high-risk sexual behavior, not sharing needles, is the biggest predictor of HIV infection in males and females who inject drugs.

"In the past, we assumed that injecting-drug users (IDUs) who were HIV-positive had been infected with the virus [that causes AIDS] through needle-sharing," said Dr. Steffanie Strathdee, associate professor of epidemiology at the Johns Hopkins University Bloomberg School of Public Health, who led the study, financed by the National Institute on Drug Abuse (NIDA).

"Our analysis indicates that sexual behaviors, which we thought were less important among IDUs, really carry weight in terms of risks for HIV for both men and women," the researcher, in Barcelona this week for the U.N. International AIDS Conference, told the publication NIDA Notes.

The research, which tracked 1,800 people living in Baltimore who had a history of injection-drug use and who were all HIV-negative when they entered the study, found that homosexual activity was the most important risk factor in HIV transmission for men.

Incidence of HIV among males 18 and older who said they had injected drugs and had engaged in homosexual activity within the previous six months was 10.4 percent per year. That was more than three times the infection rate (3.0 percent) for men who did not report having homosexual sex.

High-risk heterosexual activity was the leading cause of HIV among injection-drug-using women who took part in the study. The annual rate of HIV infection among the female participants who said they recently had a sexually transmitted disease was 8.1 percent, 2.5 times greater than the HIV-infection rate for those who did not have an STD.

More than 90 percent of study participants said they had injected drugs in the six months prior to enrolling in the research project.

They were interviewed every six months about their recent drug use and sexual behavior, and they submitted blood samples to determine if they had become HIV-positive since their last visit.

In the early days of HIV/AIDS, IDUs were referred to as intravenous drug users. But that term is now "politically incorrect," said Angela Grant, director of HIV prevention services for the Addiction Research and Treatment Corp. in Brooklyn, N.Y. She said the old term made it appear as though HIV was transmitted only by "mainlining," or shooting directly into a vein, rather than "through injection or puncturing the skin."

Dr. Strathdee's study also found that risky drug-use practices were strong predictors of HIV transmission for men but were less significant for women.

For example, men who said they had visited "shooting galleries," where drug abusers gather to obtain and use drugs, had an HIV incidence rate of nearly 6.3 percent per year. Men who shared needles with more than one partner had an HIV incidence rate of 5.5 percent per year.

Those infection rates were more than double those found among men who had not engaged in those behaviors. The Hopkins study also found that men who had injected drugs at least once daily had HIV infection rates of nearly 4.7 percent yearly, more than one and a half times the rate among men who had injected less than once a day.

While the HIV infection rate was 8.1 percent per year for women in the Hopkins study who had recently contracted STDs, the rates were 4.4 percent and 4.7 percent, respectively, for women who had shared needles and those who had visited shooting galleries.

Dr. Strathdee noted that "both homosexual men and heterosexual women IDUs appear to be at dual risk for becoming infected with HIV."